The present invention relates to a grinding machine and, more specifically, to a surface grinding machine adapted to grind a surface of a workpiece, such as a semiconductor wafer, having a very small thickness, for example, of several hundreds .mu.m to 1 mm (1,000 .mu.m).
In general, semiconductor devices are manufactured through the process of forming many elements on a thin plate which is called a semiconductor wafer, cutting the wafer into chips, and enclosing the chips with containers. In this manufacturing process, the wafer is the main object of handling. However, the wafer is made of, for example, a single crystal silicon that is brittle and is easily broken by handling in the manufacturing process. Moreover, with the progress of semiconductor technology, the outer diameter of the wafer tends to be increased in order to reduce the manufacturing cost by mass production and, at present, is a great as 4 inches or more. The greater the outer diameter of the wafer is, the more the wafer tends to be easily broken, and accordingly the wafer has to be maintained thick to a certain extent.
On the other hand, if a thick wafer is cut and manufactured into semiconductor devices, the conductivity of heat is poor and the electric characteristics are adversely affected. It is therefore necessary to adjust the thickness of the wafer by grinding the back surface of the wafer during the manufacturing process. Furthermore, in the process of forming the semiconductor elements on the wafer, the back surface of the wafer is formed with diffusion layers, as well as various layers of aluminium, polycrystalline silicon, silicon dioxide, phospho silicate glass and the like, which are achieved by deposition and heat treatment. However, the back surface of the wafer is as important to the semiconductor device as the side surface of the wafer, on which semiconductor elements are formed, from the viewpoint of taking-out electrodes, uniform heat radiation from the device, and so forth. Accordingly, even if there is no need to adjust the thickness, it is necessary to remove the layers as mentioned above. Furthermore, for easy soldering, i.e. mounting of the chip, it is required to finish the back surface of the wafer to a surface having a reasonable surface roughness.
For providing the above mentioned adjusting of the thickness and finishing of the surface, there has been used a method in which the back surface of the wafer is subjected to etching with chemicals. This method, however, requires a large quantity of chemicals, resulting in increased manufacturing cost. Furthermore, handling the chemicals is dangerous, and the disposal of the used chemicals is a troublesome problem from the viewpoint of environmental pollution.
Under these circumstances, grinding machines adapted for grinding a thin plate have been devised and used. In a conventional machine, however, there are various problems which will become apparent from the description set forth below.
A typical grinding machine known in the art is schematically illustrated in FIGS. 1 and 1A of the accompanying drawings, in which FIG. 1 is a plan view and FIG. 1A is a sectional view taking along line A--A in FIG. 1. In these Figures, the reference numeral 1 denotes a rotating table of about 800 mm in diameter, which rotates in the direction of the arrow "X". The table 1 is made of stainless steel and is provided with a plurality of workpiece holders 2 which are constructed by embedding porous ceramic plates in the table. Wafers 3 are placed on the holders 2 with the back surface up and are to be held in place by vacuum suction illustrated by the arrow "V" in FIG. 1A. Above the table 1 is disposed a grinding wheel 4, which is mounted on a spindle (not illustrated) and rotates at a speed of about 2,400 rpm in the direction of the arrow "X" and grinds successively the back surfaces of the wafers 3 by using diamond grains adhered onto the lower surface of the wheel 4. If the diamond grains have a grain size of 1,200 mesh, the wafer 3 is ground by a thickness of about 2 .mu.m when the table rotates once. Therefore, in the case of grinding a thickness of 100 .mu.m, for example, the table 1 has to be rotated 50 times, for which an operation time of over ten minutes is usually required. Such a long time consuming grinding operation makes it difficult to provide an automatic manufacturing system for continuous mass production of semiconductor devices.
In the illustrated conventional machine, when the wafers 3 are removed from the table 1 after the completion of the grinding operation, the vacuum suction "V" is interrupted and, successively, air is injected to the holders 2, as illustrated by the dotted arrow "W" in FIG. 1A. The injected air serves to facilitate the removal of the wafer and, also, to clean away fine particles on the surfaces of the holders 2, that are produced by the grinding operation. In this case, because of the flatness of the table 1, it is required to clean the entire surface of the table 1. It is, however, difficult to clean completely the entire table surface having a large area. Accordingly, when a new wafer, that is to be ground in the next operation, is positioned on the holder 2, residual fine particles are sandwiched in between the holder 2 and the wafer and this causes microcracks on the surface of the wafer, i.e. the device side surface on which the semiconductor elements are formed. Consequently, the semiconductor elements are damaged. Moreover, there is also a risk that the wafers will be carried away together with the injected air toward the circumference of the table and will be superposed upon each other.
Furthermore, in a grinding machine of this sort, a preparatory operation, which is called a dressing operation, is frequently required to ensure a good degree of parallelism for the workpiece. The dressing operation is performed by grinding the surfaces of the workpiece holders 2 to provide a good degree of parallelism thereof. In the illustrated conventional machine, however, because of the evenness of the holders with the table, it is impossible to provide a good degree of parallelism of the holders, unless the table 1 is also ground simultaneously with grinding the holders 2. In this case, the grinding of the table 1 made of stainless steel requires the use of a grinding wheel adapted for stainless steel, which is different from a grinding wheel adapted for a wafer. Consequently, the dressing operation is complicated and inefficient. Moreover, unlike porous ceramics, stainless steel has a large thermal expansion coefficient, that makes it difficult to provide a good degree of parallelism of the holders.
Furthermore, in the illustrated conventional machine, the holders 2 are embedded in the table 1 and are not exchangeable. Therefore, in order to adapt the machine to grind wafers having various diameters, it is required to prepare tables which are provided with holders having various diameters, and to exchange the tables according to the sizes of the wafer.